Modern wireless networks commonly employ CDMA techniques to communicate information between a mobile terminal and base station. Modulating information using CDMA techniques provides an advantage over other modulation methods because CDMA techniques enable multiple base stations to simultaneously use the same channel space to communicate information. Thus, CDMA techniques permit channel overlap between base stations, which has a number of significant advantages in wireless communication systems including the reduction of interference between mobile terminals and base stations, the exploitation of wireless network multipath components, and the simultaneous modulation and demodulation of information on multiple channels with multiple base stations.
Soft handoff is one method that uses these advantages to reduce data error and increase the quality of service for wireless CDMA networks. Soft handoff is a steady-state condition wherein a mobile terminal simultaneously communicates identical information with a plurality of base stations. Soft handoff increases transmission and reception diversity at the mobile terminal and mobile switching center of the wireless CDMA network, thereby increasing information capacity and quality of service while reducing the requisite signal to noise power ratio necessary to communicate information. Soft handoff typically exists throughout a mobile terminal's network connection; nonetheless, the plurality of base stations that communicate with the mobile terminal may change as the mobile terminal physically changes its location, thereby requiring the mobile terminal to switch the base stations with which it communicates.
In order to implement soft handoff within a wireless CDMA network, a mobile terminal and the plurality of base stations it communicates with must perform certain functions in order to maintain the plurality of mobile terminal-base station network connections. First, the mobile terminal must receive multiple base station transmissions on the forward link from the base station to the mobile terminal, and then combine these transmissions to aggregate the information sent by the plurality of base stations. This aggregation reduces the information error rate and increases the quality of service for the mobile terminal. Thus, the base stations must use identical CDMA symbols to modulate information and synchronize their transmissions to the mobile terminal on the forward link for the mobile terminal to accurately aggregate and demodulate the transmissions received from the base stations.
In addition, the plurality of base stations receives multiple mobile terminal transmissions on the reverse link from the mobile terminal to each base station. A mobile switching center aggregates or selects appropriate transmissions from the multiple transmissions received by the base stations in order to reduce the error rate and maintain a sufficient quality of service. Thus, the mobile terminal must also synchronously transmit information modulated with identical CDMA symbols to the plurality of base stations for the mobile switching center to aggregate or select CDMA information received from the mobile terminal.
Wireless CDMA voice networks are one common example of a wireless CDMA network that employs soft handoff. In a wireless CDMA voice network, a plurality of base stations is networked together through a common mobile switching center, which connects the base station network to the public phone network. The base stations communicate with mobile terminals using layer 2 Radio Link Protocol (RLP) frames, which include CDMA-modulated information communicated between the base stations and mobile terminals. For the forward link, the plurality of base stations synchronously transmits identical voice information that has been modulated using identical CDMA modulation symbols. This identical, synchronous, CDMA-modulated voice information is received by the mobile terminal, which aggregates the received information and modulation symbols to combine the information received and demodulate the voice information. Although the received information will vary in power, delay, and quality due to variances within the plurality of base station channels, aggregation enables the mobile terminal to recover the voice information and provide a greater quality of service.
For the reverse link, the mobile terminal modulates voice information using CDMA coding symbols and transmits the CDMA-modulated information to the plurality of base stations simultaneously. The base stations receive the CDMA-modulated information and forward the information to a mobile service center, which selects or aggregates the information received to maximize the quality of service and overcome transmission errors associated with information received at any single base station. The mobile service center then demodulates the CDMA-modulated information to recover the voice information transmitted by the mobile terminal.
A number of features about wireless CDMA voice networks make it relatively easy to implement soft handoff of mobile terminals. First, voice information requires a relatively small bandwidth, thereby permitting the mobile terminal and mobile switching center to employ simple combination and interpolation techniques to demodulate and recover voice information. Second, the CDMA-modulated voice information communicated between the plurality of base stations and the mobile terminal is identical, and thus the information received at both the mobile terminal and the mobile switching center can be combined and selected using relatively simple techniques.
In contrast, the features of wireless CDMA IP networks are markedly different from wireless CDMA voice networks, and make employment of simple aggregation, interpolation, and selection techniques virtually impossible. First, IP information can have a large and variable bandwidth that varies with the mobile terminal, IP application communicating information, and type of information being communicated. Second, wireless IP information communicated between each base station and mobile terminal is not identical because although the message content transmitted is identical, the IP header information for each message varies according to the IP subnet address of the base station communicating with the mobile terminal.
Modern wireless IP architectures use IP-based protocols to communicate messages in data packets between mobile terminals and base stations. In these architectures, IP base stations are connected to a wireless IP backbone network through edge routers, which interface the base stations with the backbone network. Each IP base station performs dual functions as both an RF front end for base station-mobile terminal communications and as a real-time router for IP data packets communicated between the mobile terminals and the wireless IP backbone network. The wireless IP backbone network further includes an IP-based control plan to route messages to and from the base station network that communicates with mobile terminals.
Variations of wireless IP architectures include architectures wherein each base station's coverage area defines an IP subnet. In these “all-IP” architectures, each base station has its own IP subnet address, and each base station and mobile terminal require layer 3 mobility management techniques to communicate messages between the base station and mobile terminal as the mobile terminal crosses a cell boundary. Thus, a mobile terminal may require an IP subnet address change when it crosses from one cell site to another using Mobile IP, DHCP, DRCP, or other layer 3 mobility management techniques.
One solution to the problems presented by soft handoff of mobile terminals in wireless “all-IP” CDMA networks is IP multicasting. Briefly, IP multicasting is a process wherein information is simultaneously communicated to a plurality of multicast group members. Each multicast group and its members are identified by an IP multicast address, which identifies the multicast group, and hence multicast group members, to which a multicast message should be communicated.
Soft handoff via IP multicasting occurs by creating an IP multicast address associated with a mobile terminal. Each mobile terminal-base station communication link is then assigned as a multicast group member to the mobile terminal's multicast group as identified by the mobile terminal's IP multicast address. Information is then communicated between the mobile terminal and its plurality of base stations used for soft handoff by broadcasting the information to the mobile terminal's multicast group. The information is thereby communicated via the plurality of mobile terminal-base station communication links simultaneously, thereby establishing soft handoff of the mobile terminal.
Although soft handoff of a mobile terminal via IP multicasting provides an elegant solution to providing soft handoff of a mobile terminal in wireless CDMA IP networks, it is difficult to combine or select a “best” CDMA IP data frame from the plurality of IP CDMA data frames received as part of the IP multicast process. In particular, the IP data packets communicated between the plurality of base stations and a mobile terminal in a soft handoff situation are not identical due to their different header information, which is attributable to the different IP subnet addresses for the base stations. Thus, the IP information communicated between the plurality of base stations and the mobile terminal cannot be aggregated according to prior art methods for voice data wherein the voice information communicated between each base station and mobile terminal is identical. As a result, there is a need for an efficient packet aggregation or selection mechanism that will create a “best” IP data packet from the plurality of redundant IP data packets received from different base stations despite their different header information.
In addition, this problem of packet aggregation and selection extends to all other methods of CDMA IP data frame communication independent of IP multicasting wherein a plurality of redundant IP data packets must be combined into a single “best” CDMA IP data packet to perform soft handoff of a mobile terminal. For these other methods, there remains a high demand for an efficient packet aggregation or selection mechanism that will create a “best” IP data packet from a plurality of redundant IP data packets communicated between a mobile terminal and a plurality of base stations.